This invention deals with an improved method of controlling a high speed electric motor, which has a front-end Electromagnetic Interference (EMI) filter attached. By extracting estimated voltage and current values of the motor windings based on sensed power inverter output values, a controls scheme that is more accurate than the prior art controls schemes is obtained.
In industrial applications, regular electric motors are often driven by variable frequency power source. A variable frequency power source is used because in many motors the frequency of the input power controls the speed of the motor. A variable frequency power inverter converts Direct Current (DC) power into Alternating Current (AC) power. Variable frequency power inverters have the capability to adjust the frequency and voltage of the AC power. This allows the power inverter to control the speed and torque of an AC motor attached to it by adjusting both the frequency and voltage of its power output. In most applications the power inverter is connected directly to the motor with no intermediate EMI filter connections. This practice is common for motors of all sizes in most operations and provides the motor with standard power, though not clean power. Clean power is the power that is provided relatively free of electronic and electromagnetic noises and is typically achieved through the use of an EMI (Electro-Magnetic Interference) filter.
Some operations, for example aerospace and aircraft applications among others, have very limited electrical tolerances and thus require clean power. Clean power is achieved once a filter has been applied and electrical noise has been filtered off of the power transmission. To achieve clean power in the present setting it is necessary to introduce an EMI filter between the power inverter and the motor winding terminals. Since EMI filters are often set up in complicated inductance-capacitance formats, the electrical values (such as current, voltage, and impedance) of the EMI filter in high power, high frequency, and high speed applications become complex issues as the power level output of the power inverter increases. Simultaneously the electrical values of the motor parameters decrease as the power level output of the power inverter increases. As a result the electrical values of the EMI filter become substantially large relative to the electrical values of the motor parameters. This results in the output voltage and current of the power inverter being significantly affected by the EMI filter.
In the prior art methods of controlling motors in this configuration the voltage and current values at the output of the power inverter have been sensed and then erroneously used to control the motor by assuming them to be the voltage and current values at the input of the motor windings. Feedback controls, such as the ones used in the prior art, work by measuring the value of the voltage and current at the power inverter output and assuming them to be the voltage and current values of the motor, ignoring the effect that the EMI filter has on the voltage and current values. The controller then adjusts the input to the power inverter accordingly using the normal pulse-width modulation switching methods (i.e. depending on the load demands, the power inverter switching pattern can be varied to increase or decrease the output voltage and frequency applied into the motor inputs) The influence of the EMI filter on the voltage and current values increases the amount of time it takes to correct any improper voltage and current values in the motor control system, and it may give some disturbance response during transient due to an imbalance between the supply and demand sides of the electric machine. The duration of time necessary to correct the values is referred to as the response time. Erroneously assuming the power inverter output values to be the actual motor winding values results in a controls scheme that is slowed down or sluggish, as well as less accurate than desired.